Problem 31 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An example of a reversible reaction is : (a) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \mathrm{aq}+2 \mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{PbI}_{2}(\mathrm{~s})+2 \mathrm{NaNO}_{3}(\mathrm{aq})\) (b) \(\mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}(\mathrm{aq})\) (c) \(2 \mathrm{Na}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(1) \rightarrow 2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})\) (d) \(\mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq}) \rightarrow \mathrm{KCl}(\mathrm{aq})+\mathrm{NaNO}_{3}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Reaction (d) is reversible.

Step 1: Identify Reversible and Irreversible Reactions

A reversible reaction is one where products can react to form the original reactants. Conversely, an irreversible reaction proceeds in one direction only, where the products do not revert back into reactants.

Step 2: Analyze Reaction (a)

The reaction is: \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \mathrm{aq}+2 \mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{PbI}_{2}(\mathrm{s})+2 \mathrm{NaNO}_{3}(\mathrm{aq}) \). In this reaction, the solid \( \mathrm{PbI}_{2} \) precipitates out of solution, indicating an irreversible reaction.

Step 3: Analyze Reaction (b)

The reaction is: \( \mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}(\mathrm{aq}) \). Here, \( \mathrm{AgCl} \) precipitates out of the solution, also indicating an irreversible reaction.

Step 4: Analyze Reaction (c)

The reaction is: \( 2 \mathrm{Na}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(1) \rightarrow 2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g}) \). This involves the production of a gas (\( \mathrm{H}_{2} \)) and is not easily reversible.

Step 5: Analyze Reaction (d)

The reaction is: \( \mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq}) \rightarrow \mathrm{KCl}(\mathrm{aq})+\mathrm{NaNO}_{3}(\mathrm{aq}) \). All substances are in aqueous form and can theoretically undergo reverse reactions under suitable conditions. This reaction is reversible.

Key Concepts

Chemical Equilibrium

Chemical equilibrium refers to a state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction. This dynamic balance means that although reactions are still occurring, the overall concentrations of reactants and products remain constant over time. In a closed system, without any additional changes like temperature or pressure shifts, the reactions will persist with no net change.
Reactions that reach equilibrium can be described using the equilibrium constant, denoted as \(K_{eq}\). This constant is a ratio of the concentrations of products to the concentrations of reactants, each raised to the power of their respective coefficients in the balanced chemical equation.

• A large \(K_{eq}\) indicates a greater concentration of products at equilibrium.
• A small \(K_{eq}\) suggests a higher concentration of reactants.

Thus, in reversible reactions like \(\mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq}) \rightleftharpoons \mathrm{KCl}(\mathrm{aq})+\mathrm{NaNO}_{3}(\mathrm{aq})\), the dynamic nature of chemical equilibrium allows for both forward and backward processes to occur simultaneously, reflecting the balancing act that defines chemical equilibrium.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions are mixed together, and an insoluble solid, known as a precipitate, forms. This is an irreversible process because the precipitate has different properties from the original reactants, often meaning it cannot readily revert back to them without further chemical change.
The formation of a precipitate can usually be predicted by using solubility rules, which help determine whether combining particular ions in solution will produce an insoluble compound. It’s important to note that not all reactions between aqueous solutions result in precipitation.

• Reactions such as \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \mathrm{aq}+2 \mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{PbI}_{2}(\mathrm{s})+2 \mathrm{NaNO}_{3}(\mathrm{aq})\) involve the formation of a solid precipitate (\(\mathrm{PbI}_{2})\), signifying that the products will not easily go back to a dissolved state.
• This is in contrast to reactions where no solid forms, which may remain reversible under certain conditions.

Aqueous Solutions

Aqueous solutions are solutions where water is the solvent. They are central to understanding many chemical reactions, especially those occurring in nature and industrial processes. In these solutions, solutes (the dissolved substances) break into ions, which can freely move and react with other ions or molecules present.
The behavior of ions in aqueous solutions is fundamental in reactions such as precipitation and acid-base processes. The solvent ability of water is incredibly high due to its polarity and capacity to stabilize diverse ions by surrounding them, an action known as hydration.

• For instance, in the reaction \(\mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}(\mathrm{aq})\), both reactants are initially dissolved in water, allowing them to collide and react effectively.
• Such reactions are common in lab settings and natural environments because of the prevalence of water as a medium for reactions.

Understanding the characteristics and behaviors of aqueous solutions is pivotal in predicting and explaining the outcomes of chemical reactions.